Preliminary data from out laboratory have indicated that virulent and avirulent strains of Legionella pneumophila differ in their sensitivity to the bactericidal activity of complement. Virulent strains are not sensitive to the alternative complement pathway and, in the absence of antibody, do not bind C3. In the presence of antibody, virulent strains bind C3 and the terminal complement components are deposited on the surface of the cell. However, the membrane attack complex apparently does not insert into the membrane since there is no reduction in the number of cells in the presence of antibody and complement. In contrast avirulent cells are sensitive to the bactericidal activity of the alternative complement pathway. The addition of antibody results in only slightly increased levels of bactericidal activity. Based on these preliminary data the specific aims of this proposal are to: 1) separate virulent and avirulent cells by use of FACS sorting; 2) determine the stability of pure populations of virulent and avirulent cells and investigate the mechanism of the observed shift; 3) determine the binding site of C3 on avirulent cells It is anticipated that these studies will provide significant information concerning possible virulence factors and may help to elucidate environmental factors that influence the virulence of L. pneumophila.